1. Field of the Invention
The invention relates to a display device of optical shutter type which changes the quantity of light entering an aperture by means of changing the size of a droplet of a coloring liquid by utilization of an electrowetting phenomenon, and more particularly, to a technique for promoting restoration of spherical shape of the droplet of a coloring liquid.
2. Description of the Related Art
A liquid-crystal display device (LCD), a plasma display (PDP) device, a guest-host LCD, an electro-chromic display (ECD), an electrophoresis display (EPD), an organic EL display (OLED) and the like have hitherto been known as related-art display devices.
However, these known display devices suffer many problems in terms of luminance, contrast ratio, resolution, screen size, an increase in definition, responsiveness, lifetime, display of gradation, and manufacturing costs. None of them is suitable for use as a medical display which satisfies all requirements; i.e., high luminance (1000 cd/m2), high contrast ratio (1000:1), high definition (200 ppi), and a large area (800×1200 mm).
Among these display devices, the LCD is superior. However, the LCD employs a polarizing plate, and hence black luminance becomes high for reasons of leakage of light. Further, the LCD has dependence on a viewing angle (hereinafter called a “viewing angle dependency”), which in turn poses a problem in utilization of the display device by many people and gives rise to eye strain. In the case of a display device of spontaneous illumination, such as a PDP or an OLED, neither sufficient luminance nor a sufficient contrast ratio is obtained, thereby posing difficulty in achieving uniform luminance over the entire screen (i.e., a problem of luminance variations). If uniform luminance is achieved by rendering the entire screen bright, the display device will suffer from a problem of short lifetime. Moreover, difficulty is encountered in rendering the definition of the PDP high.
As described in JP-A-9-311643 and JP-A-10-39800, a display device of optical shutter type has already been known, wherein the quantity of light entering an aperture is changed by means of changing the size of a droplet of a coloring liquid by utilization of the electrowetting phenomenon.
JP-A-9-311643 and JP-A-10-39800 relate to a display utilizing an electrowetting phenomenon (in the publications the phenomenon is referred to as an “electrocapillarity”).
JP-A-9-311643 offers a method for manufacturing an electronic display sheet. The electronic display sheet is formed from: first and second sheets, each having an external surface and an internal surface; and a sealed space existing between the internal surfaces of the first and second sheets. The first sheet has electrode means of first type provided on the internal surface of the sheet, an insulation layer disposed over the electrode means of first type, and electrode means of second type which is placed on top of the insulation layer and exposed to the sealed space. The insulation layer is formed and arranged to isolate the electrode means of first type from the electrode means of second type. The electronic display sheet further comprises means for impelling small droplets placed in the sealed space covering the electrode means of second type and energizing the electrode means of first and second types. When the electrode means of first and second types are energized, droplets are enlarged.
JP-A-10-39800 describes an electrocapillary display sheet utilizing a plurality of sets of conductive droplets of a coloring liquid within a space existing between two sheets. Respective sets of droplets of a coloring liquid are allocated corresponding electrodes provided on the respective sheets. A droplet of a coloring liquid belonging to each set is not blended with another droplet of a coloring liquid in the same set. Respective droplets of a coloring liquid possess electrical connection, and droplets of a coloring liquid belonging to the respective sets are selectively activated. As a result, at least one droplet of a coloring liquid in one droplet set spreads into a space shared by the droplets of a coloring liquid belonging to the respective sets, thereby forming color pixels of an image.
In any of these display devices, spread of the droplet of a coloring liquid is accelerated by the force of an electric field. However, the droplet of a coloring liquid returns by means of only a water repellent characteristic, and hence returning of the droplet becomes slow, thus deteriorating responsiveness. Hence, display of animation is difficult.
In addition, according to the related-art technique, the low-surface energy film, which is included in the insulation layer, is imparted with an insulation characteristic. Hence, in order to change the contact angle, the low-surface energy film must be made thin. However, a reduction in the thickness of the low-surface energy film results in leakage of an electric current, thus failing to achieve sufficient insulation performance.
Specifically, according to the related-art technique, the insulation layer formed by combination of the insulation film and the low-surface energy film has a thickness on the order of microns. The insulation film and the low-surface energy film are formed from a single material in an integrated fashion. A PTFE [polytetrafluoroethylene: Teflon (trade name)]-based material having specific inductive capacity of 2.1 is used. Changing the area of a droplet by changing the contact angle θ through several degrees requires a voltage of 100 V or more. Thus, the related-art technique is not practical.
In any of these display devices, spreading of a droplet of coloring liquid is poor, and the display device has a low numerical aperture, thus deteriorating utilization efficiency of light.